Previously, French Pat. No. 2,202,899 disclosed the homopolymerization of ethylene and the copolymerization of ethylene with at least one .alpha.-olefin, continuously, in the presence of a Ziegler-type catalytic system, at high temperature and under high pressure.
Apparatus of the '899 French Patent comprised at least one reactor having at least one reaction zone, at least one separator, and one or more conduits for recycling the unreacted monomers of ethylene and .alpha.-olefin, if any, towards a secondary compressor. The secondary compressor receives fresh monomers from a primary compressor, and the secondary compressor also feeds monomers into the reactor at the chosen polymerization pressure.
In a process of this type, traces of the components of the catalytic system can be carried over with the gases. This carry-over can cause a continuation of the polymerization both before and during the separation of the polymers or copolymers also formed from the unreacted monomers. Such a carry-over can cause a reduction in the average molecular weight of the polymers or copolymers, which are collected in the separator.
The continuation of the polymerization or the copolymerization in either the conduit connecting the reactor to the separator or in the separator itself is very inconvenient, because the continued polymerization requires the unpredictable adjustment of the operating settings of the reactor. Similarly, in the separator, the polymerization can cause the appearance of a thermal gradient between the upper and lower parts of the separator. Consequently, it is very difficult, under these conditions, to obtain a polymer or a copolymer having the required properties.
The carry-over of the traces of the components of the catalytic system can, moreover, cause the polymerization of the monomers that are recycled in the recycle conduits from the separator. The polymers or copolymers that are formed, generally are of very low molecular weight. Typically, the polymers or copolymers have a molecular weight below 2,000, and have the appearance of oils, greases, or waxes, under normal conditions of temperature and pressure.
Traps are generally provided in the recycle conduits to collect the polymers or copolymers of very low molecular weight. However, these polymers or copolymers can also be deposited on the inner wall of the recycle conduits, and their molecular weight can increase as a consequence of the continuous flow of the recycle gases, which contain traces of the components of the catalytic system. Although the polymers or copolymers of very low molecular weight cause little inconvenience since they are easy to remove, the polymers or copolymers of higher molecular weight, having a molecular weight greater than 2,000, are a major nuisance since they can cause a pressure drop in the apparatus and even result in the blocking of the recycle conduits.
French Pat. No. 2,302,305 describes a polmerization process that injects, at the end of the reaction, at least one product selected from the alkali metal salts or alkaline earth metal salts of carboxylic acids. The quantity of the product that is injected is sufficient to cause the deactivation of the catalyst. The catalyst, preferably, has both a transition metal derivative and an activator constituent. The injection of the metal salt takes place, preferably, close as possible to the reactor outlet valve.
The process of the '305 French Patent is not totally satisfactory, because a thermal gradient appears in the separator. Although the thermal gradient is slightly reduced, it is still too high. Furthermore, the concentration of but-2-ene formed by the isomerization of but-1-ene is also too high. The presence of but-2-ene is particulary inconvenient, because it does not copolymerize with the ethylene and the .alpha.-olefins. Consequently, the but-2-ene accumulates in the recycled gases.
French Pat. Nos. 1,267,771 and 1,268,693 disclose processes for preparing, at a low pressure and a low temperature, polymers of vinyl hydrocarbons. A catalyst is used that comprises an organic compound of a metal from groups IA to IIIA and an organic compound of a metal of group IIB, such as Zn, Cd, Hg, and Mg. The organic compound of a metal of Group IIB acts as a co-activator.
German Pat. No. 2,841,646 describes a process for interrupting and restarting the polymerization and copolymerization of olefins at a low pressure and a low temperature, by adding at least 0.5 millimole of CO or, preferably, CO.sub.2 per liter of reaction mixture. This process is applied to the polymerization or copolymerization of olefins in the presence of a catalytic system comprising a titanium trichloride modified by an electron donor and a chlorodialkylaluminum. The restarting of the polymerization is possible, and it is found that, under the conditions described, carbon monoxide is not a deactivator of the catalyst.